1. Field of the Invention
The present invention relates to an illumination optical system, an exposure apparatus, and a method of manufacturing a device.
2. Description of the Related Art
To improve the resolution of an exposure apparatus, the RET (Resolution Enhancement Technology) or modified illumination methods such as annular illumination and quadrupole illumination are commonly employed. Especially in an EUV (Extreme Ultraviolet) illumination optical system, the intensity of light which reaches a substrate decreases with an increase in number of mirrors, so an aperture stop is used to extract light instead of providing a mirror unit for modified illumination. In doing so, the amount of light beams shielded by the aperture stop is increased, resulting in illumination with low efficiency. FIG. 18 in Japanese Patent Laid-Open No. 11-312638, for example, describes an embodiment in which an EUV light beam from a light source enters fly-eye mirrors 20a and 20b without being uniformized into a parallel light beam by a mirror 15. In this embodiment, aperture stops 50a to 50f corresponding to desired modified illumination are inserted in front of the fly-eye mirror 20b to partially shield the light beam, thereby implementing modified illumination. In such a configuration, nonuniformity of the angle distribution of a light beam from the light source is directly reflected on the annular intensity distribution, so the uniformity of the effective light source distribution degrades, thus adversely affecting the resolution performance.